When rolling sheet steel in a so-called cold-rolling mill to reduce its thickness and give the steel the desired finish, the rolling process inevitably stretches the strip longitudinally and thereby deforms the strip from its original planarity. Complex mechanical procedures have been suggested to bias the last pair of working rolls together in such a way as to roll out the deviations from planarity. Such procedures normally are, however, quite crude and cannot react rapidly to temporary deviations from planarity in the workpiece to correct them during the run.
It has been suggested in German patent document No. 2,908,641 based on a Dutch application filed Mar. 6, 1978 by H. K. Quere and A. J. Tychon to flatten aluminun strip by thermally changing the diameter of regions of the last working rolls in the aluminum-strip mill. To this end the strip passes as it exits from the last roll stand over a sensing device subdivided into a plurality of zones spaced apart on the strip transverse to the strip displacement direction and set up to generate respective output signals corresponding to the deviations from planarity of the strip at these zones. Respective streams of a treatment liquid are sprayed at regions of the rolls of the last stand corresponding to the zones to effect the required slight diameter change. If the rolls need to be of larger diameter they are cooled less so that they heat up and expand, and vice versa. In this manner an extremely fine control of the roll diameters can be obtained starting from conventional cylindrical rolls. The heat exchange can be varied by changing the temperature of the treatment liquid or its rate of flow.
Such a procedure cannot, however, be used with steel strip, even though, or actually because, it is standard practice to spray the working rolls in a steel mill with a treatment liquid that serves the functions of lubricating, cleaning, and even cooling. This liquid is normally an oil/water emulsion that must be delivered to the working rolls at a certain temperature and at a certain rate that is a function of volume of treatment liquid per unit of surface area of the working roll per unit of time. Normally the liquid is relatively warm--40.degree.-50.degree. C.--for proper emulsification and chemical action. If the temperature of the emulsion is not right the viscosity and droplet size will be less than or greater than what is needed, or any detergent or other surface active agent in the treatment liquid will be less effective than intended. Changing the rate of flow can lead to dangerous underlubrication of the rolling process whereas increasing it can cause slippage.